1. Field of the Invention
The present invention relates generally to systems for producing polyolefins. More particularly, it concerns a system for controlling the temperature of an olefin polymerization reactor.
2. Description of Related Art
Olefins, particularly alpha-olefins, also referred to as 1-olefins, have several uses. For example, alpha-olefins are used in polymerization processes, either as monomers or comonomers, to prepare polyolefins. Many commercial methods are available to produce olefin polymers, such as polyethylene. One of the most economical routes for making commercial grades of olefin polymers is a loop/slurry process. In a loop/slurry process, the reactor contains a slurry that includes solid catalyst, polymer and liquid monomers. The slurry is circulated in a loop, with some product being removed from the loop periodically or continuously.
The temperature at which the polymerization is carried out is important in determining the molecular weight of the polymer produced, and in preventing the polymer from swelling, which can cause fouling of the polymerization reactor. Furthermore, if a diluent (e.g., paraffin) is used in the loop/slurry process, the polymerization process can be carried out at a temperature low enough that the resulting polymer is largely insoluble in the diluent, simplifying the recovery process.
Methods of the present invention are directed to controlling the temperature of an olefin polymerization reactor system. The olefin polymerization reactor system comprises a polymerization reactor and a cooling jacket that is in thermal contact with the reactor. In certain embodiments the olefin polymerization reactor is used to polymerize ethylene to produce a homopolymer, or to polymerize ethylene and at least one other alpha-olefin to produce a copolymer. In some embodiments the at least one other alpha-olefin comprises from four to twelve carbon atoms. In certain embodiments the olefin polymerization reactor is a loop reactor, especially a loop/slurry reactor. Methods of the present invention can be directed to controlling the temperature of (e.g., removing heat from) any known loop reactor.
In some embodiments aqueous froth is supplied to the cooling jacket, and in other embodiments a downflow aqueous solution can be supplied to the cooling jacket. The aqueous froth can be supplied to the cooling jacket as a downflow in certain embodiments. In certain embodiments the aqueous froth is supplied to the cooling jacket as an upflow. In certain embodiments the aqueous froth as it is supplied to the cooling jacket comprises between about 95% to 99% by weight liquid, and in some cases between about 97% and 99% by weight liquid. The pressure in the jacket is lower than atmospheric pressure, and heat is transferred from the reactor to the jacket, causing at least some of the water in the aqueous froth or downflow aqueous solution to vaporize, forming a vapor phase and a first liquid phase. The vapor phase and the first liquid phase are separated at a pressure that is lower than atmospheric pressure, and at least a fraction of the vapor phase is condensed to form a second liquid phase at a pressure that is lower than atmospheric pressure. A vacuum pump can be used to maintain the pressure in the system at a level that is lower than atmospheric pressure. The precise pressure in the system can be adjusted so as to maintain a desired temperature in the reactor.
In certain embodiments, the second liquid phase and the first liquid phase are combined to produce a collected recycle water stream, which can be recycled to the jacket. In certain embodiments, before the collected recycle water stream is recycled to the jacket it can be superheated to between 10 and 15 degrees Fahrenheit over the temperature at which the collected recycle water stream boils when at the pressure present in the cooling jacket.
Certain embodiments of the present invention are directed to a method for controlling the temperature of an olefin polymerization reactor system that includes a polymerization reactor, and a cooling jacket in thermal contact with the reactor. At least one alpha-olefin having from two to twelve carbon atoms can be polymerized in the reactor. The at least one alpha-olefin can consist of ethylene or ethylene and, for example, 1-butene, 1-hexene, or 1-octene. An aqueous froth is supplied to the cooling jacket. The pressure in the jacket can be lower than atmospheric pressure, and the heat transferred from the reactor to the jacket causes at least some of the water in the aqueous froth to vaporize, thereby forming a vapor phase and a first liquid phase. The vapor phase and the first liquid phase can be separated at a pressure that is lower than atmospheric pressure. At least a fraction of the vapor phase can be condensed to form a second liquid phase at a pressure that is lower than atmospheric pressure. The pressure in the system can be maintained by a vacuum pump, and the pressure in the jacket can be adjusted so as to maintain a desired temperature in the reactor. The second liquid phase and the first liquid phase can be combined, thereby producing a collected recycle water stream. The collected recycle water stream can be superheated to between 10 and 15 degrees Fahrenheit over the temperature at which the collected recycle water stream boils when at cooling jacket pressure. The collected recycle water stream can be recycled to the jacket.
Certain embodiments of the present invention can permit improved reactor temperature control. Certain reactor temperature control methods of the present invention can result in lower capital costs than methods known in the art. In certain embodiments of the present invention the reactor can have an improved heat transfer coefficient relative to known reactor cooling schemes that employ single liquid phase cooling.